The present invention relates generally to a process for removing copper which is present in boron doped silicon wafers and, more particularly, to a process for diffusing copper from the bulk to the surface of such wafers from which it can be removed while avoiding the formation of copper precipitates. Typically, boron doped silicon wafers have a boron concentration sufficient to obtain a resistivity of less than about 100 ohms-cm.
Integrated circuit manufacturers generally require that the concentration of copper on the surface of a silicon wafer be no more than 1.times.10.sup.10 atoms/cm.sup.2 to 1.times.10.sup.11 atoms/cm.sup.2, as determined by methods standard in the art. It is foreseeable that this requirement will be decreased to a value of 5.times.10.sup.9 atoms/cm.sup.2, 1.times.10.sup.9 atoms/cm.sup.2 or less since a large fraction of random device failures can be traced to copper silicide precipitates. Under appropriate conditions, copper reacts with silicon to form a copper silicide precipitate, sometimes referred to as haze defects because, upon being subjected to a common etching treatment and bright light inspection, such defects appear as a haze on the surface of the wafer.
In the past, it has been observed that chemomechanical polishing of boron doped, p type silicon wafers results in a temporary increase in electrical resistivity of the wafers. Early research into this phenomenon suggested that this effect was the result of acceptor neutralization by hydrogen, which was being incorporated into the silicon matrix of the wafers. It was noted in this regard that, upon being heated to 180.degree. C. for 30 minutes, the initial acceptor activity in the wafers was mostly restored. (See Schnegg et al., Mat. Res. Soc. Symp. Proc., 104 (1988), pp. 291-96.) Recently, however, studies have shown that the increase in resistivity is the result of copper, as opposed to hydrogen, being incorporated into the wafers during the chemomechanical polishing process. (See, e.g., Prigge et al., J. Electrochem. Soc., 138 (1991), pp. 1385-89.) This is a surprising discovery because wafer polishing typically occurs at room temperature. The solubility of copper in a silicon wafer is so low at room temperature that any copper present should typically be found on the surface.
Referring now to FIG. 1, immediately after wafers have undergone conventional polishing treatments and state of the art cleaning methods, the copper concentration on the surface of the wafers is less than 1.times.10.sup.10 atoms/cm.sup.2, as determined by total reflection spectroscopy (TXRF) measurements. (See, e.g., C. Neumann et al., Spectrochemica Acta, 10 (1991), pp. 1369-1377; Ingle & Crouch, Spectrochemical Analysis, Prentice Hall, 1988.) The surface copper concentration of these wafers, however, increases with the passage of time even at room temperature until saturation is reached. Thus, wafers which meet a target specification for copper surface concentration immediately after cleaning may fail to meet this specification as soon as five to ten months later.
Gettering has been used in the past in an attempt to trap copper and other metals and prevent these contaminants from reaching the device region of the wafer. Such an approach has not proven to be entirely effective for copper, however, due to the high diffusivity of copper in silicon which makes it possible for copper to escape from the gettering sites and reach the device region of the wafer.
Prigge et al. (DE 3939661 A1) proposed a process to address the problem of copper incorporation during wafer polishing in which an attempt is made to limit the amount of copper actually incorporated into the wafer. This is accomplished by means of admixing with polishing agents certain reagents which form a coordination complex with copper. This coordination complex acts to maintain the copper in a specific conformation which limits the ability of copper to enter the silicon.
While the amount of copper incorporated into the wafers varies, depending upon the many factors associated with available polishing methods, no method has been found which is completely free of this effect. Therefore, a need continues to exist for a process which allows for the removal of copper from polished, boron-doped silicon wafers.